1. Field of the Invention
This invention relates to nickel/titanium shape memory alloys and improvements therein.
2. Discussion of the Prior Art
Materials, both organic and metallic, capable of possessing shape memory are well known. An article made of such materials can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The article is said to have shape memory for the reason that, upon the application of heat alone, it can be caused to revert, or to attempt to revert, from its heat-unstable configuration to its original, heat-stable configuration, i.e. it "remembers" its original shape.
Among metallic alloys, the ability to possess shape memory is a result of the fact that the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change in temperature. This transformation is sometimes referred to as a thermoelastic martensitic transformation. An article made from such an alloy, for example a hollow sleeve, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation begins is usually referred to as the M.sub.s temperature. When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to austenite, referred to as the A.sub.s temperature, the deformed object will begin to return to its original configuration.
Shape memory alloys have found use in recent years in, for example, pipe couplings such as are described in U.S. Pat. Nos. 4,035,077 and 4,198,081 to Harrison and Jervis, and electrical connectors such as those described in U.S. Pat. No. 3,740,839 Otte and Fischer, the disclosures of which are incorporated by reference herein.
These alloys also find use in switches, such as are disclosed in U.S. Pat. No. 4,205,293, and actuators, etc. For such application, it is generally desirable that the A.sub.s temperature should be above ambient, so that the alloy element will remain in its martensitic state unless heated either externally or by the passage of an electric current through it. Because of the hysteresis of the austenite-martensite transformation, the desired M.sub.50, the temperature at which the transformation to martensite is 50% complete, will will generally be above 0.degree. C. for an A.sub.s above, say, 20.degree. C.
Especially in the case of switches, actuators, and heat engines, in which the shape memory alloy element may be subject to repeated cycling between the austenitic and martensitic states under load, shape memory "fatigue" may be a problem. Cross et al, NASA Report CR-1433 (1969), pp. 51-53, discuss briefly this phenomenon, which they term "shape recovery fatigue", and indicate that there may be a significant loss in recovery at higher strain levels for binary nickel-titanium.
For shape memory applications in general, a high austenitic yield strength is desirable, as this minimizes the amount of the somewhat expensive alloy required and the size of the article.
Various alloys of nickel and titanium have in the past been disclosed as being capable of having the property of shape memory imparted thereto. Examples of such alloys may be found in U.S. Pat. No. 3,174,851 and 3,351,463.
Buehler et al (Mater. Des. Eng., pp. 82-3 (February 1962); J. App. Phys., v. 36, pp. 3232-9 (1965)) have shown that in the binary Ni/Ti alloys the transformation temperature decreases dramatically and the yield strength increases with a decrease in titanium content from the stoichiometric (50 atomic percent) value. However, lowering the titanium content below 49.9 atomic percent has been found to produce alloys which are unstable in the temperature range of 100.degree. C. to 500.degree. C., as described by Wasilewski et al., Met. Trans., v. 2, pp. 229-38 (1971). The instability (temper instability) manifests itself as a change (generally an increase) in M.sub.s between the annealed alloy and the same alloy which has been further tempered. Annealing here means heating to a sufficiently high temperature and holding at that temperature long enough to give a uniform, stress-free condition, followed by sufficiently rapid cooling to maintain that condition. Temperatures around 900.degree. C. for about 10 minutes are generally sufficient for annealing, and air cooling is generally sufficiently rapid, though quenching in water is necessary for some of the low Ti compositions. Tempering here means holding at an intermediate temperature for a suitably long period (such as a few hours at 200.degree.-400.degree. C.). The instability thus makes the low titanium alloys disadvantageous for shape memory applications, where a combination of high yield strength and reproducible M.sub.s is desired.
Certain ternary Ni/Ti alloys have been found to overcome some of these problems. An alloy comprising 47.2 atomic percent nickel, 49.6 atomic percent titanium, and 3.2 atomic percent iron (such as disclosed in U.S. Pat. No. 3,753,700 to Harrison, et al.) has an M.sub.s temperature near -100.degree. C. and a yield strength of about 70,000 psi. While the addition of iron has enabled the production of alloys with both low M.sub.s temperature and high yield strength, this addition has not solved the problem of instability, nor has it produced a great improvement in the sensitivity of the M.sub.s temperature to compositional change.
U.S. Pat. No. 3,558,369 shows that the M.sub.s temperature can be lowered by substituting cobalt for nickel, then iron for cobalt in the stoichiometric alloy. However, although the alloys of this patent can have low transformation temperatures, they have only modest yield strengths (40,000 psi or less).
U.S. Naval Ordnance Laboratory Report NOLTR 64-235 (August 1965) examined the effect upon hardness of ternary additions of from 0.08 to 16 weight percent of eleven different elements to stoichiometric Ni/Ti. Similar studies have been made by, for example, Honma et al., Res. Inst. Min. Dress. Met. Report No. 622 (1972), on the variation of transformation temperature with ternary additions.
U.S. Pat. No. 4,144,057 shows that the addition of copper to NiTi alloys containing traces of at least one other metal produces alloys in which the transformation temperature is relatively less dependent on the composition than it is in the binary alloys. Such a control of transformation temperature is referred to in U.S. Pat No. 4,144,057 as "stabilization". This use of "stabilization" should be distinguished from the use made by the present applicant, who, as stated before, uses "stability" to refer to freedom from change of transformation temperature with conditions of manufacture.
Two further requirements for these shape memory alloys should be noted. These are workability and machinability. Workability is the ability of an alloy to be plastically deformed without crumbling or cracking, and is essential for the manufacture of articles (including even test samples) from the alloy. Machinability refers to the ability of the alloy to be shaped, such as by turning or drilling, economically. Although machinability is not solely a property of the alloy, Ni/Ti alloys are known to be difficult to machine (see, e.g., Machining Data Handbook, 2nd Ed. (1972) for comparative machining conditions for various alloys), i.e. they are expensive to shape, and a free-machining nickel/titanium shape memory alloy would be extremely economically attractive.
While U.S. Pat. No. 4,144,057 shows that control of transformation temperature with composition may be achieved by the addition of copper, it does not suggest compositions or conditions which produce alloys having good stability (as defined above), workability, and machinability: all of which properties are important for the economic manufacture of memory metal articles.
In particular, U.S. Pat. No. 4,144,057 is directed principally towards alloys containing sufficient titanium that ternary addition is not required for temper stability. Further, it fails to distinguish between those elements which are believed to assist in providing temper stability, e.g. Al and Zr, and those which do not, e.g. Co and Fe.
As stated in my U.S. Pat. No. 4,377,090, I have discovered that the addition of copper to nickel/titanium alloys having a low transition temperature (an A.sub.50, the temperature at which the transformation to austenite is 50% complete, in the range of from -50.degree. C. to -196.degree. C.) provides a significant improvement in temper stability, enabling the production of high yield strength, low M.sub.s alloys.